A circuit including a clamp circuit and a measuring capacitor for automatically setting the black level voltage at the cathode of a picture tube is known from DE-OS 35 35 570. During the time interval which is not utilized for graphic display, the measuring capacitor is charged, at each frame cycle, by the cathode current. The obtained measured value is received by a scanning and holding circuit and is made available to the control circuit for the remaining frame duration. The control circuit supplies the black level voltage at which the cathode potential is brought into contact with the clamp circuit.
In P 38 19 214.4, a circuit is known for setting the mean brightness of a picture represented with a video signal by holding constant the mean value of the video signal. The mean brightness of image fragments are controlled at a setpoint which is independent from the gray scale value of the remaining image. To accomplish this, the video signal and the signal of a setpoint device are each carried through one switch which is closed during the time periods when the video signal of the image fragments appears. The output signals of both switches are supplied via integrators having equal time constants to a controller. The controller output is coupled to the control input of a controllable attenuator lying in the video signal path. A controllable amplifier can also be used in place of the controllable attenuator. A rectifier is arranged upstream of the integrator. Instead of using the mean value of the video signal, the peak value or another value can also be held constant.
Above all, this type of amplitude control is suited for the video recording of continuously illuminated or luminous images. If the images are illuminated in a pulsewise manner, i.e., by light pulses having varying widths and intervals such as in X-ray video monitors, then the video signal amplitude is controlled according to the resulting mean value of the pulse/interval ratio. However, this type of amplitude control is often undesirable.